477
FXUS63 KLOT 171714
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1114 AM CST MON FEB 17 2025
KEY MESSAGES
- BITTERLY COLD CONDITIONS ARE EXPECTED THROUGH THURSDAY
MORNING. COLD WEATHER ADVISORIES ARE IN EFFECT FOR THIS
MORNING AND AGAIN FOR TUESDAY MORNING.
- THERE IS ABOUT A 30-40% CHANCE FOR LAKE EFFECT SNOW TO
ACCUMULATE IN PORTIONS OF PORTER COUNTY, IN TUESDAY THROUGH
WEDNESDAY.
- THERE IS ANOTHER CHANCE (20-40%) OF FLUFFY ACCUMULATING SNOW
AND ASSOCIATED TRAVEL IMPACTS AREAWIDE LATE TUESDAY NIGHT
THROUGH WEDNESDAY NIGHT.
- TEMPERATURES WILL GRADUALLY WARM BACK TOWARD AND EVEN ABOVE
NORMAL FOR THIS TIME OF YEAR EARLY NEXT WEEK.
DISCUSSION
ISSUED AT 340 AM CST MON FEB 17 2025
THROUGH TUESDAY:
THE MAIN STORY HERE IN THE NEAR TERM (AND REALLY, PRETTY MUCH
THIS WHOLE WORKWEEK) IS THE BITTERLY COLD CONDITIONS THAT HAVE
ARRIVED AS A LOBE OF THE POLAR VORTEX HAS DESCENDED INTO THE
UPPER MIDWEST AND UNLEASHED A CHILLY AIR MASS INTO THE REGION.
AT PRESS TIME, TEMPERATURES ACROSS THE AREA WERE IN THE SINGLE
DIGITS, AND WITH A FEW MORE HOURS OF UNINHIBITED NOCTURNAL
RADIATION IN THIS DRY AIR MASS AMIDST CONTINUED MODEST LOW-LEVEL
COLD AIR ADVECTION, EXPECT TEMPERATURES TO FALL A FEW DEGREES
MORE BEFORE SUNRISE. SUB-ZERO LOW TEMPERATURES ARE THUS STILL
EXPECTED ACROSS INTERIOR NORTHERN ILLINOIS, WHERE SUSTAINED
WEST-NORTHWESTERLY WINDS AROUND 10 MPH WILL LEAD TO WIND CHILLS
AS COLD AS -20 TO -25F, SO MADE NO CHANGES TO THE GOING COLD
WEATHER ADVISORY FOR THIS MORNING.
DAYTIME TEMPERATURES ONLY LOOK TO REBOUND INTO THE MID-SINGLE
DIGITS TO MID-TEENS TODAY, COLDEST FARTHER NORTHWEST AND
"MILDEST" FARTHER SOUTHEAST, AS MODEST COLD AIR ADVECTION
CONTINUES AND CLOUD COVER BEGINS TO INCREASE. WHILE WINDS TODAY
WON'T BE AS STRONG AS THEY WERE YESTERDAY, THEY WILL STILL PICK
UP A BIT AFTER SUNRISE AND REMAIN BLUSTERY ENOUGH FOR SUB-ZERO
WIND CHILLS TO PERSIST ACROSS BASICALLY THE ENTIRETY OF OUR
FORECAST AREA ALL DAY LONG.
WHILE THE PRESENCE OF CLOUD COVER MAY PREVENT MANY LOCATIONS
FROM REALIZING OPTIMAL RADIATIONAL COOLING CONDITIONS THROUGH
MUCH OF TONIGHT, THE COLD STARTING POINT TO THE NIGHT, THE
DRYNESS OF THE AIR MASS (SURFACE DEW POINTS FALLING BELOW -10F
IN MANY LOCATIONS AND 800-850 MB DEW POINTS LOWER THAN -40F
DEPICTED IN FORECAST SOUNDINGS), AND CONTINUED MODEST COLD AIR
ADVECTION OFF OF NORTHWESTERLY WINDS WILL LIKELY LEAD TO TONIGHT
INTO TUESDAY MORNING BEING THE COLDEST PERIOD OF THIS MID-
FEBRUARY COLD SPELL. LOW TEMPERATURES FOR THIS PERIOD ARE
PRESENTLY FORECASTED TO FALL BELOW 0 ACROSS ALMOST ALL OF
NORTHERN ILLINOIS (AS COLD AS -5 TO -10F IN INTERIOR PORTIONS OF
THE STATE), AND THEY WON'T BE MUCH BETTER IN OUR NORTHWEST
INDIANA COUNTIES. WITH SUSTAINED WINDS GENERALLY REMAINING
AROUND 10 MPH OR SO, WIND CHILLS WILL BOTTOM OUT BETWEEN 15 AND
30 BELOW ZERO ACROSS OUR FORECAST AREA, AGAIN COLDEST FARTHER
NORTHWEST AND "MILDEST" FARTHER SOUTHEAST.
AS A RESULT, HAVE ISSUED ANOTHER COLD WEATHER ADVISORY FOR
TONIGHT/TUESDAY MORNING FOR LOCATIONS WHERE WIND CHILLS OF -20F
OR LOWER ARE EXPECTED, WHICH IS BASICALLY IN ALL OF OUR ILLINOIS
COUNTIES/FORECAST ZONES. CONFIDENCE IS ONLY LOW TO MEDIUM IN
THIS -20F THRESHOLD BEING REACHED IN OUR ILLINOIS LOCALES NEAR
AND SOUTH OF I-80 AND WITHIN THE URBAN HEAT ISLAND-INFLUENCED
CENTRAL COOK COUNTY, BUT FELT THAT THE COURSE OF LEAST REGRET
WAS TO TOSS THESE LOCATIONS IN THE ADVISORY ANYWAYS. WILL ALSO
ISSUE A SPECIAL WEATHER STATEMENT FOR THE COLD FOR OUR NORTHWEST
INDIANA COUNTIES, WHERE MINIMUM WIND CHILLS ARE PRESENTLY TO
END UP BELOW OUR COLD WEATHER ADVISORY CRITERIA.
THERE WILL ALSO BE A COUPLE OF CHANCES FOR SNOWFLAKES TO BE
OBSERVED TODAY AND TUESDAY ALONG THE FRINGES OF OUR FORECAST
AREA. FIRST, A SWATH OF SNOW TIED TO MODEST MID-LEVEL
FRONTOGENESIS AND DPVA TRACKING ACROSS THE LOWER MISSOURI RIVER
VALLEY WILL PROGRESS EASTWARD INTO CENTRAL AND SOUTHERN ILLINOIS
LATER TODAY/TONIGHT. THE MORE COHERENT FORCING LOOKS TO REMAIN
SOUTH OF OUR FORECAST AREA, AND WITH DRY LOW- TO MID-LEVEL AIR
NEEDING TO BE OVERCOME, THE PROSPECTS OF ACCUMULATING SNOW
ACROSS OUR SOUTHERNMOST COUNTIES SEEM FAIRLY LOW OVERALL.
NONETHELESS, FORECAST SOUNDINGS FROM SOME MODELS STILL SUPPORT
AT LEAST FLURRIES OCCURRING SOUTH OF A ROUGHLY LASALLE/PERU, IL
TO RENSSELAER, IN LINE, SO OPTED TO ADD A FLURRY MENTION TO THE
FORECAST GRIDS FOR THESE LOCATIONS.
ADDITIONALLY, LATE TONIGHT AND ESPECIALLY INTO TUESDAY, LOW-
LEVEL WINDS OVER LAKE MICHIGAN WILL GRADUALLY VEER FROM A WEST-
NORTHWESTERLY TO A MORE NORTH-NORTHWESTERLY DIRECTION. THIS WILL
CAUSE MULTI-BANDED LAKE EFFECT SNOW OCCURRING IN WESTERN
MICHIGAN TO SHIFT INTO NORTHWEST INDIANA, AND LIKELY INTO AT
LEAST NORTHEASTERN PORTIONS OF PORTER COUNTY. FOR NOW, HAVE
BOOSTED POPS AND SNOW ACCUMULATIONS UPWARD HERE JUST A TOUCH,
BUT COULD FORESEE FURTHER UPWARD ADJUSTMENTS BEING NEEDED IF
SOME OF THE CAMS HOLD STEADY WITH THEIR QPF OUTPUT.
OGOREK
TUESDAY NIGHT THROUGH SUNDAY:
BITTERLY COLD TEMPERATURES ARE EXPECTED TO CONTINUE THROUGH
MIDWEEK WITH DAYTIME HIGH TEMPERATURES IN THE TEENS BOTH
WEDNESDAY AND THURSDAY AND OVERNIGHT LOWS IN THE -5 TO 5 DEGREE
RANGE. WHILE NOT AS COLD AS EARLIER IN THE WEEK, WIND CHILLS
COULD STILL DROP INTO THE -10 TO -20 DEGREES IN SPOTS DURING THE
OVERNIGHT AND EARLY MORNING HOURS.
IN ADDITION TO THE COLD, WE ARE STILL MONITORING THE POTENTIAL
FOR ACCUMULATING SNOW DURING THE TUESDAY NIGHT THROUGH
WEDNESDAY TIMEFRAME. A PACIFIC NORTHWEST ORIGINATING SHORTWAVE
TRAVERSING THE SOUTH CENTRAL PLAINS MAY ATTEMPT TO PARTIALLY
PHASE WITH AN UPPER LEVEL POLAR LOW EXPECTED TO MOVE ACROSS THE
UPPER MIDWEST. THERE CONTINUES TO BE SOME VARIABILITY IN
GUIDANCE AS TO WHETHER THE AREA WILL SEE SNOWFALL (OUTSIDE OF
ANY LAKE EFFECT IN NW INDIANA), WHICH WILL DEPEND ON HOW FAR
SOUTH THE SOUTHERN SHORTWAVE TRACKS/DEGREE OF PHASING WITH THE
UPPER LOW. MODEL SOUNDINGS INDICATE THAT THERE WILL ALSO BE A
SUBSTANTIAL LAYER OF DRY AIR IN THE LOW-LEVELS THAT WILL HAVE TO
ERODE TO ALLOW SNOW TO REACH THE SURFACE. ABOVE THIS DRY LAYER,
THERE IS A DEEP SATURATED LAYER WITH ISOTHERMAL TEMPERATURES
RESIDING ALMOST ENTIRELY WITHIN (OR ON THE UPPER END OF) THE
DGZ. IF THE DRY LAYER CAN BE OVERCOME (POTENTIALLY AIDED BY
DOWNSTREAM LAKE EFFECT MOISTURE), A QUICK COATING OF FLUFFY SNOW
WOULD OCCUR. WHILE INDIVIDUAL MODEL RUNS HAVE VARIED AS TO
WHETHER WE STAY DRY OR NOT, 0Z ENSEMBLES STILL HAVE A ROUGHLY
40-60% CHANCE OF MEASURABLE SNOW (>=0.1") OCCURRING GENERALLY
ALONG AND SOUTH OF I-55. ACCORDINGLY, OPTED TO MAINTAIN 20-40%
CHANCES FOR SNOW AREAWIDE DURING THE TUESDAY NIGHT THROUGH
WEDNESDAY TIMEFRAME. GIVEN HOW COLD TEMPERATURES WILL BE, ANY
SNOW THAT FALLS WOULD READILY ACCUMULATE ON PAVEMENT LEADING TO
SLICK TRAVEL.
THE CENTER OF THE EXPANSIVE SURFACE HIGH WILL BEGIN TO SHIFT
SOUTHEAST OF THE AREA TOWARD THE END OF THE WEEK. THIS WILL
BRING A RETURN TO SOUTHWESTERLY FLOW AND A VERY GRADUAL
MODERATION OF TEMPERATURES BACK TO NORMAL BY EARLY NEXT WEEK
(1991-2020 CLIMATE NORMALS FOR LATE FEBRUARY IN CHICAGO ARE IN
THE UPPER 30S TO AROUND 40).
PETR
AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/
ISSUED AT 1113 AM CST MON FEB 17 2025
- NO SIGNIFICANT CONCERNS THROUGH TUESDAY AFTERNOON.
BREEZY WEST/WEST-NORTHWEST WINDS WITH GUSTS TO 20-25 KT THIS
AFTERNOON WILL VEER TO NORTHWESTERLY THIS EVENING AND REMAIN
NORTHWESTERLY (PRIMARILY 300-330 DEG) THE REST OF THE PERIOD.
THERE MAY BE GUSTS TO AROUND 20 KT AT TIMES THIS EVENING AND
OVERNIGHT. ASIDE FROM LAKE EFFECT CLOUDS EAST OF THE NEAR LAKE
TERMINALS, A FEW TO SCATTERED CLOUDS AT 2500-3000 FT MAY DEVELOP
THIS AFTERNOON AND AGAIN ON TUESDAY AFTERNOON. WHILE SPORADIC
BKN HIGH MVFR CIGS CAN'T BE COMPLETELY RULED OUT, CONDITIONS
WILL BE PRIMARILY VFR.
CASTRO
LOT WATCHES/WARNINGS/ADVISORIES
IL...COLD WEATHER ADVISORY FROM 9 PM THIS EVENING TO NOON CST
TUESDAY FOR ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 3 PM CST THIS AFTERNOON FOR THE IL
AND IN NEARSHORE WATERS.
VISIT US AT WEATHER.GOV/CHICAGO
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